FIRE-CLAY, FIRE-BRICKS. Fire-clays may be defined as native combinations of hydrated silicates of alumina, mechanically associated with silica and alumina in various states of subdivision, and sufficiently free from silicates of the alkalies and from iron and lime to resist vitrification at high temperatures ; the absence of the vitrifiable element is, however, merely a question of degree, as no native clays are wholly free from iron, the alkalies, lime, and the other alkaline earths.
Fire-clay may be looked upon as a special term for the grey clays of the Coal-Measures, interstra-tified with, and generally in close proximity to, the seams of coal, in beds varying from a few inches to many yards in thickness. They are locally known as " chinches " and " underclays," and are supposed to represent the soil that produced the vegetation from which the coal was formed.
The association of coal with the fire-clays of the carboniferous formation has localized the manufacture of firebricks, and by far the larger proportion are produced in the Coal-Measure districts, especially at Stourbridge, celebrated for producing a highly refractory brick, Broseley, Benthall, Madeley and Coalbrookdale in the Shropshire coal-field, and in the Midland, Yorkshire, North and South Wales,Durham, and the Scotch coalfields; but in later years the area of fire-brick manufacture has much widened. There has been an extensive production since about 1850 from the Eocene clays in the neighbourhood of Poole and Wareham in Dorsetshire ; and a more limited supply from the Miocene between Bovey Tracey and Newton Abbot in Devorslaire.
Still more recently Cornwall has become the seat of the manufacture, where, as at Calstock, Tregoning Hill near Breage, St Ednor near St Columb, and Lee Moor, fire-bricks of fine quality are made from china-clay refuse and disintegrated granite. Mr Argall of the Tregoning Hill Company states that the locality was one of the The foregoing article is reprinted by permission of Messrs Little, Brown, & Co, 'Boston, Mass., from Great Fires and Fire Extinction, by General Alfred P. Rockwell, Boston, 1878.
first seats of china-clay mining between the years 1730 and 1750, and that in 1862 the present company commenced to make fire-bricks and tiles from the refuse of the clays, taking about two-thirds of silica and one-third of mica, which are mixed together in a pug mill, moulded and burnt in round ovens holding about 16,000 bricks, and that a very superior fire-brick is made from clay direct from the "stopes," containing which are employed by founders, smelters, gas companies, &c. The price paid at the works is from 50s. to 55s. per 1000. The source of the materials is decomposed granite, of which Tregoning Hill consists.
The Hingston Down fire-clay deposit-, near Calstock, supplying the Calstock fire-brick works, the Phoenix works, and the Tamar works in the same neighbourhood, consists of a range of decomposed granite with an average width of three-quarters of a mile, running east and west for 3 or 4 miles, extending to an ascertained depth of from 300 to 400 feet, and intersected by mineral lodes. The Calstock Fire-Brick Company (limited), superintended by Mr C. B. Evate, commenced operations in the year 1870, and manufactured from the decomposed granite fire-bricks of a highly refractory character, which are delivered free on board at the port of Calstock at from 50s, to 60s. a thousand, weighing about 3/ tons. Another source of firebrick material, scarcely yet developed, is the pockets or depressions occurring in the mountain limestone of North Wales, Derbyshire, and Ireland, containing white refractory clays and sands, the insoluble remnants from the local dissolution of the limestone, intermixed with the debris of the overlying millstone grit. These clays and sands when evenly mingled are sufficiently adhesive to be moulded, and their small contractility and highly refractory character render them pre-eminently suitable for fire-brick manufacture. Fire-brick works have already been established on the estate of Captain Cooke of Colomeudy Hall, near Mold, and the refractory clays and sands are largely employed for lining furnaces, 3000 tons having been sold for this purpose alone in the year 1877.
The fire-clays of the Coal-Measures vary as regards their refractory character, not only in the different coal-fields, but the individual strata iu close alternation often present sudden variations, refractory beds being interstratified with useless strata largely charged with disseminated carbonate of iron. The grey colour of the Coal-Measure clays is partially due to the presence of this mineral, which, whether disseminated through the mass or otherwise occurring in excess as concretionary nodules, is prejudicial to the clays as a material for fire-bricks. Carbonaceous matter is also present in variable proportions, colouring the clay from a slaty-black to a pale grey, but as this is eliminated in the earlier stages of the burning of the bricks, its presence in no way influences their refractory character.
The relative proportion of silica and alumina which some manufacturers have laid undue stress upon as indicating heat-resisting quality is of little moment, as both these constituents, whether occurring in combination as silicates of alumina, or as free alumina and silica, are essentially the refractory elements of all good fire-bricks, being unvitrifiable per se, excepting when associated with the alkalies, lime, or oxides of iron. The plastic character of refractory clays is also of limited influence on their suitability for fire-brick manufacture; extreme plasticity, which is generally accompanied by excessive contractility and vitrifiability, is prejudicial. As a rule few clays or materials used in the manufacture of fire-bricks are insufficiently plastic to prevent their being easily moulded, and in the manufacture by the dry process, or the compression of the brick out of nearly dry pulverized clay, plasticity is not so essential a quality. As regards chemical composition the following analyses indicate its general character, and the variability of the proportion of the constituents of sonic of the more important fire-clays : -
The above analyses indicate a preponderance of silica in the Coal-Measure fire-clays compared with the Tertiary clays of Devon and Dorset, in which a larger proportion of alumina occurs. The latter character is accompanied by tenacity and plasticity, and greater contraction in drying and burning, which, when excessive, is counteracted by mingling with the clay sand and ground burnt clay or sherds.
Coietractility. - The contraction in burning (excluding the contraction in drying) of several Coal-Measure fire-clays pressed out of nearly dry pulverized clay was ascertained by the writer to be as follows: - Fire-clay occurring; between the "Penystene Measure" ) The average contraction of the Coal-Measure fire-clays is thus little over two per cent. A brick manufactured from the best Stourbridge clay without admixture of burnt material contracted seven-eighths of an inch in nine inches, but this would probably represent the contraction both in drying and burning. The contraction iu burning of the Tertiary fire-clays used for the manufacture of fire-bricks in Devon and Dorset is much more than that of the Coal Measure clays The clays above enumerated are not exclusively used for fire-brick manufacture, but fairly indicate the general character of the Tertiary fire-brick clays, which, compared with the Coal-Measure fire-clays, are characterized by a preponderance of alumina, tenacity of texture, contractility in the kiln, and an absence of iron and the alkalies, &c., which tend to vitrification. Tenacity of texture iu a fire-brick material is, however, a mechanical condition, which, cceterie paribus, assists vitrification, a coarse open body being more refractory than a close homogeneous brick of similar chemical composition. A well manufactured brick should be of a pale cream or clear buff colour, uniform throughout its mass, and burnt to the full extent of its contractility.
The chemical changes which take place in the burning consist, first, of the destruction of the disseminated carbonaceous matter, the dehydration of the silicates of alumina, destroying their plastic character, and the decom= position of the disseminated carbonate of protoxide of iron, converting it into anhydrous sesquioxide, to which the yellow colour of the burnt brick is due ; if the burning is carried to a high state of vitrification the yellow tint is replaced by a dull grey, due to the partial reduction of the sesquioxide of iron and its conversion into silicate of protoxide or minutely disseminated particles of metallic iron; any alkalies present also form vitreous combinations with the silica during the latter stages of the burning. The paleness of colour of a fire-brick is not always a safe indication of the absence of iron, as the presence of a large proportion of carbonaceous matter in the clay tends to bleaching by the reduction of the colouring sesquioxide to a lower oxide preserved as a silicate in a comparatively colourless condition. Again, the presence of lime and the other alkaline earths, which are disadvantageous fluxing elements, will cloak the colouring paver of a large percentage of oxide of iron by the formation of a pale double silicate of lime and iron. This is taken advantage of in the manufacture of buff building bricks by mixing ground chalk with ferruginous clays which would otherwise burn dark red.
A properly burnt brick, uniform in colour throughout its mass, can only be obtained by slow progressive firing ; a broken brick that has been too quickly burnt, though pale on the surface, presents a darker central patch and concentric rings of various shades of colour, due mainly to the different states of oxidation of the iron, and partly to the presence of unconsumed carbonaceous matter ; but the chemistry of this colour-variegation is not clearly understood.
Durability of Fire-Bricks.. - The destruction and wearing out of a fire-brick in its ordinary uses takes place in different ways. First, it may waste by crumbling and shattering ; this occurs only when the brick is unnecessarily porous in texture, or from the presence of extraneous lumps of foreign Materials, such as small pebbles and fragments of lime and iron-stone, which the manufacturer endeavours to get rid of by sifting or crushing before the clay is moulded. Secondly, the gradual vitrification of the brick under the pressure of the superimposed structure distorts its form, and the semiplastic red-hot or white-hot MISS is gradually squeezed out of shape, and has to be periodically replaced in the hotter parts of the furnaces and kilns. However completely a brick has been burnt, bringing its dimensions almost to the limit of contractility, constant exposure to long-continued heat still further reduces its bulk, causing the displacement of the mass of which it forms a part, and necessitating replacement and repairs. Thirdly, there is the gradual fretting away of the exposed brick surface by vitrification, however refractory a brick may be; when it lines flues and furnaces, the fumes and ashes incessantly curled into contact with it bring foreign accessions, which vitrify the exposed portions and form a coating of viscid slag, which eats into the brick surface, creeping down and clogging the flues and fire-holes with a vitreous mass. In the case of blast-furnaces the fretting away of the surface of the fire-brick linings gradually enlarges their capacity, the surface destruction decreasing from the tweers upwards, the faces opposite the impact of the blast being distinctly excavated beyond the outline of the enlarged circumference. In one of the furnaces of the Madeley Wood Company, Madeley, Shropshire, "blown in" in 1867 and " blown out" in 1874, the diameter at the base, originally 3 feet, had been enlarged to about 7 feet 6 inches, and at the widest part, a little below the middle, from 12 feet, its original diameter, to 13 feet. In another furnace, after ten years' blast., the original diameter of 4 feet 5 inches at. the base had been enlarged to 9 feet G inches; at a third of its height from 9 feet 11 inches to 12 feet; and half way up, from 11 feet 9 inches to 12 feet 10 inches, the destruction of brick-surface gradually decreasing towards the top, where the increase of size was but trifling. This would represent a consumption of brick-surface of from 3 to 4 inches a year at the tweers, and about three-quarters of an inch a year towards the middle of the furnace.
It is beyond the scope of this article to enter into the details of fire-brick manufacture, which in its main features resemble the manufacture of building bricks, except that fire-bricks are rarely if ever burnt in clamps.
Properly constructed ovens on the " down draft " principle, with the outlet from the bottom into a tall chimney, are now almost universally employed, as they ensure greater regularity in the burning than in the old form of kiln, with a direct escape from the top, as well as economy iu fuel. The consumption of coal varies from 9 to 15 cwt. per 1000 bricks, exclusive of the coal used in the drying stoves.
Coal-Measure fire-clays are often mined in an almost rocky condition, requiring long exposure to the weather to effect their disintegration. The softer clunches or clays are sometimes prepared for the moulder by "weathering," but the more common practice is to grind the fire-clay just as it comes from the pit under heavy " runners " or rollers, effecting a granular texture which is a desirable quality in a refractory brick. It is now becoming a common practice to grind up with the raw clay from one-fourth to one-third of its weight of broken burnt sherds or fire-bricks. All waste materials are thus utilized, and the excessively contractile character of highly plastic clays such as those of Dorsetshire is counteracted. Silicious sand is also sometimes mixed with the more plastic clays to reduce their contractility.
The ground clay is either brought into a plastic state with water in a pug-mill and moulded by hand, or by brick-moulding machinery generally connected with the pug-mill outlet ; or the partially moist ground-clay dust is compressed into bricks in iron moulds by steam power, a modification of Prosser's well-known process. More shapely bricks are thus produced than by plastic moulding, and their perfectly true flat sides enable a minimum of jointing materials to be employed - a circumstance of importance in the stability of fire-brick masonry, as thick fire-clay jointing contracts in the firing, tending to shatter the structure.
In addition to the use of fire-clay for the bedding of all fire-brick structures, it forms the materials of gas retorts, crucibles, and every kind of potter's kiln furniture, such as saggars, cranks, slip kiln bottoms, enamel kiln linings, &c. These have been up to within the last year or two moulded out of plastic clay, but by processes and machinery recently patented by Mr A. Maw, saggars, cranks, and every kind of kiln furniture can be moulded out of nearly dry pulverized clay, at a great reduction in cost below that of the plastic process, and the exact regularity of form attained effects a large saving of space in the ovens and kilns. Moreover, the even bearing on each other of the regularly-shaped saggars when piled up in the oven, reduces the " wear and tear" and breakage to a minimum. Fire-bricks vary much in price in the different producing districts, - about 40s. per 1000 may now be looked upon as a minimum, ranginc,'' up to 70s. and 80s., further augmented by the cost of carriage to other districts. A thousand fire-bricks weigh from 3k to 4 tons. (G.